Praise be to Allah.
Marijuana is an intoxicating substance; it is derived from
the cannabis plant ( Cannabis indica ), which is a plant that
has an intoxicating effect. Based on that, the ruling on it is
that it is haraam (prohibited), as has previously been stated
in detail and with evidence in fatwa no. 176545
With regard to mescaline, it is a substance derived from a
type of cactus that is known in north-eastern Mexico as
peyote. It is an intoxicating compound which causes
hallucinations and colourful visions.
Psilocybin is a substance derived from some mushrooms,
such as the Mexican psilocybin mushroom. This is also an
intoxicating substance, with a stronger effect than
mescaline
With regard to what you mentioned about LSD, this is
another name for lysergic acid diethylamide. It was first
synthesized in 1938 by a Swiss pharmacist called Albert
Hofmann. It is taken in micrograms because it is one of the
strongest of all hallucinogenic drugs. This drug was banned
in 1966 and was classed as an (illegal) drug because of the
negative consequences of taking it, such as suicide and so
on.
What is mentioned in the question about these drugs having
no harmful effects is to be rejected. In fact they cause
serious harm, both psychological and physical. That is
because psilocybin, mescaline and LSD are all
hallucinogenic, intoxicating substances that have a great
impact on thinking, mood and behaviour. They lead to a lot
of illusions, mental confusion, and aural and visual
hallucinations, as well as physical lethargy, schizophrenia,
and changes in perception. The one who consumes this
substance may have fits of laughter without any reason.
The symptoms experienced by the one who consumes these
drugs may also include loss of a proper sense of time and
place, and altered perception thereof. He may imagine a
minute as lasting for a lifetime, or he may see stationary
objects as if they are coming closer to him or moving
further away from him.
This is in addition to some physical symptoms, such as
nausea, excessive burping, sweating, paleness of the face,
dilation of the pupils, rapid heartbeat, and so on.
Based on that, these substances are haraam because of
this far-reaching physical harm. The Prophet (blessings
and peace of Allah be upon him) said: “There should be no
harm nor reciprocating of harm.” Narrated by Ahmad in his
Musnad (2865), Ibn Maajah (2341); classed as saheeh by
al-Albaani in Saheeh Ibn Maajah
If we assume, for the sake of argument, that the negative
impact on health of these drugs was not known, it would
still be sufficient – in order to disallow them – to know that
they are classed under the heading of substances that
cause intoxication or make one languid, so they are haram
for that reason. The Messenger of Allah (blessings and
peace of Allah be upon him) said: “Every intoxicant is
khamr and every intoxicant is haraam. Whoever drinks
khamr in this world and dies when he is addicted to it and
has not repented, will not drink it in the Hereafter.”
Narrated by al-Bukhaari (5575) and Muslim (2003).
In fact, even if we assume that the one who consumes it
does not reach the level of becoming intoxicated, even if he
were to consume a large amount of it, at least it causes the
one who consumes it to feel lethargic or languid, and
anything that has such an effect cannot be permissible,
because of the report narrated by Umm Salamah (may Allah
be pleased with her), according to which the Messenger of
Allah (blessings and peace of Allah be upon him) forbade
everything that intoxicates or makes one languid.
Narrated by Abu Dawood in his Sunan (3686); Ahmad in al-
Musnad (26634); al-Bayhaqi in as-Sunan al-Kubra (17399)
. Al-Haafiz al-‘Iraaqi said: Its isnaad is saheeh.
End quote from Fayd al-Qadeer (6/338)
Al-Khattaabi (may Allah have mercy on him) said: That
which makes one languid is any drink that causes
languidness and lethargy in the limbs, which is the
precursor of intoxication. It is forbidden to consume it lest
it be a means that leads to intoxication.
End quote from Ma‘aalim as-Sunan (4/267).
See also ‘Awn al-Ma‘bood and Haashiyat Ibn al-Qayyim
(10/91) for the commentary on this hadeeth.
The questioner’s view that these drugs “have some
positive outcomes” does not change the shar‘i ruling in the
slightest. Even if we were to assume that this is true,
khamr – which is the mother of all evils – also has some
benefits. Allah, may He be glorified and exalted, says
(interpretation of the meaning): “They ask you (O
Muhammad) concerning alcoholic drink and gambling. Say:
‘In them is a great sin, and (some) benefits for men, but
the sin of them is greater than their benefit’” [al-Baqarah
2:219] . So Allah, may He be glorified, confirms that there
are some benefits in it, yet despite that the prohibition on it
is emphatic, because of the negative and harmful
consequences that outweigh any benefits.
Ibn Katheer (may Allah have mercy on him) said: Allah,
may He be glorified, says “ Say: ‘In them is a great sin, and
(some) benefits for men”. As for the sin, it affects religious
commitment. As for the benefits, they are worldly, because
there are some physical benefits, as it aids digestion, leads
to expulsion of waste, makes one more alert, and is very
pleasurable, as Hassaan ibn Thaabit said before he became
Muslim: “We drink it and it makes us kings and lions who
do not fear meeting the enemy.”
But these benefits cannot equal its harmful and negative
effects, because they impact both mind and religious
commitment. Hence Allah said “ but the sin of them is
greater than their benefit ”.
End quote from Tafseer Ibn Katheer (1/579). See also
Tafseer al-Qurtubi (3/57)
The scholars of the Standing Committee stated that qat is
haram, even though there may be some benefits in
consuming it; that is only because its negative effects
outweigh any benefits. They said: Even if we assume that
there are some benefits in this qat, the definite harmful and
negative consequences outweigh, and are many times
greater than, any benefit there may be in it.
End quote from Fataawa al-Lajnah ad-Daa’imah 1 (22/162)
To sum up:
What is mentioned in the question about these substances
not being harmful is not correct; rather they are indeed
harmful, which you may find out by consulting specialists
or specialist medical websites.
Moreover, they indeed lead to negative consequences, the
most serious of which is intoxication or languidness, which
dictates that they should be prohibited.
And Allah knows best.